Re: [PATCH]mm: fix-up zone present pages

[wujianguo <wujianguo106@gmail.com>]

On 2012-8-21 11:55, Petr Tesarik wrote:
> Dne Po 20. srpna 2012 08:38:10 wujianguo napsal(a):
>> From: Jianguo Wu <wujianguo@huawei.com>
>>
>> Hi all,
>> 	I think zone->present_pages indicates pages that buddy system can
>> management, it should be:
>> 	zone->present_pages = spanned pages - absent pages - bootmem pages,
>> but now:
>> 	zone->present_pages = spanned pages - absent pages - memmap pages.
>> spanned pages：total size, including holes.
>> absent pages: holes.
>> bootmem pages: pages used in system boot, managed by bootmem allocator.
>> memmap pages: pages used by page structs.
> 
> Absolutely. The memory allocated to page structs should be counted in.
> 

Hi Petr,
	Bootmem pages include the memory allocated to page structs.

Thanks!
Jianguo Wu

>> This may cause zone->present_pages less than it should be.
>> For example, numa node 1 has ZONE_NORMAL and ZONE_MOVABLE,
>> it's memmap and other bootmem will be allocated from ZONE_MOVABLE,
>> so ZONE_NORMAL's present_pages should be spanned pages - absent pages,
>> but now it also minus memmap pages(free_area_init_core), which are actually
>> allocated from ZONE_MOVABLE. When offline all memory of a zone, This will
>> cause zone->present_pages less than 0, because present_pages is unsigned
>> long type, it is actually a very large integer, it indirectly caused
>> zone->watermark[WMARK_MIN] become a large
>> integer(setup_per_zone_wmarks()), than cause totalreserve_pages become a
>> large integer(calculate_totalreserve_pages()), and finally cause memory
>> allocating failure when fork process(__vm_enough_memory()).
>>
>> [root@localhost ~]# dmesg
>> -bash: fork: Cannot allocate memory
>>
>> I think bug described in http://marc.info/?l=linux-mm&m\x134502182714186&w=2
>> is also caused by wrong zone present pages.
> 
> And yes, I can confirm that the bug I reported is caused by a too low number 
> for the present pages counter. Your patch does fix the bug for me.
> 
> Thanks!
> Petr Tesarik
>